The AO-4 Call is closed.
You can find all approved programmes here.


4th Announcement of Opportunity (AO-4)

for participation in the CHEOPS Guest Observers (GO) Programme

This 4th Announcement of Opportunity (AO-4) marks the start of opportunities in CHEOPS' first extended mission, which ESA's Science Programme Committee (SPC) has recently supported in their meeting on 7 March 2023 (read the full media release here). AO-4 opened on 4 April 2023 and will close on 25 May 2023, soliciting proposals for observations to be carried out as part of the CHEOPS Guest Observers (GO) Programme, with the observation window spanning from 25 September 2023 to 30 September 2024 (inclusive). CHEOPS offers the GO observers space-based ultra-high precision photometry for the observation of exoplanet transits, eclipses, occultations, phase-curves, and more. Science cases may range to exomoons, ring structures, stellar activity, trans-Neptunian objects, and beyond. The timely overlap of several space- and ground-based missions can provide opportunities for synergies with NASA/ESA/CSA JWST, NASA/ESA HST, NASA TESS, ESO ground-based facilities, and more.



The CHEOPS AO-4 Call comes with several novelties to further enhance the community access and GO experience:

  • More targets: only 50 reserved targets, with all the rest being open to the entire community
  • More time: up to 30% science observing time dedicated to the GO Programme
  • New approach: double-anonymous peer-review of proposals



Opening of AO-4 Call for Phase I observing proposals  4 April 2023 at 12:00 (noon) CEST
Deadline for submission of Phase I observing proposals 26 May 2023 at 12:00 (noon) CEST
Meeting of the CHEOPS Time Allocation Committee 19 - 21 June 2023
Announcement of award of observing time on CHEOPS (foreseen by) 14 July 2023
Start of preparation of Phase II inputs (if awared observing time) 21 July 2023
Start of observations of AO-4 Programmes (if awarded observing time) 25 September 2023


Important Notes 

Prospective observers must take note of the following points:

  1. Read the Policies and Procedures document carefully before starting any proposal preparation, as it is the definitive document for the AO. It contains details of CHEOPS' science programme, available time for the community, observing strategy and constraints, and required contents of observing proposals. It also lists information about the tools and documents available to support observers in their proposal preparation and submission. Also thoroughly consult the Observers Manual and all relevant documentation on this website.
  2. All proposal material and file names must be fully anonymous. This is highlighted in the Policies and Procedures document, and repeated here explicitly to leave no doubts. Failure to comply with this rule will lead to automatic rejection on formal grounds.
  3. Consult the Target Checker and Mission Archive. The CHEOPS Science Team kindly made a list of all targets (reserved and unreserved ones) as well as their programme information publicly available via these tools. The tool also gives detailed instructions on how to proceed for individual targets.
  4. Register to the CHEOPS GO Programme immediately, if you do not already have a user account. Waiting with this step until the deadline might cause you to miss the opportunity.
  5. Submit your proposal via the Proposal Handling Tool for Phase I (PHT1). Proposal submission and access to some tools requires the use of a Cosmos ID together with membership of the CHEOPS GO Programme user group (see above).

Please also note that:

  • Updates related to this AO Call and its material will be published at the bottom of this webpage.
  • The CHEOPS team will do everything they can to observe and complete awarded programmes if possible. However, it is important to appreciate that an award of observing time does not guarantee that the observations can be conducted, even if a programme is considered to be high priority science. Due to the nature of the science that CHEOPS addresses, time pressure can be high, unforeseen events can occur, and some observations might not be feasible to schedule.
  • CHEOPS is a small mission, with a correspondingly small team to support the GO Programme. Never leave proposal preparation to the very last moment, as it may take a couple of days to answer your questions.


Key Documentation


Tools and material


Additional references   



  • Note the special case of Alpha Cen (including Alpha Cen A and Alpha Cen B): The system is currently on the RTL but due to its high proper motion and orbital motion, the Target Checker will not resolve its RTL coordinates if querying by name. Alpha Cen’s RTL coordinates are given in the RTL csv file, please use these for any Target Checker queries for this system.
  • We added additional examples to help you with making your proposals dual-anonymous to the FAQ pages. Please read these carefully and do not hesitate to contact us with any questions surrounding this.

Happy proposing!


Questions about CHEOPS or the GO Programme? Please email cheops-support at and we will be happy to help!
Last updated 15 Sep 2023